Automatic gain control (AGC) apparatuses and methods have long been used in standard NTSC-type television receivers for maintaining the magnitude of tuned signals over variations in the modulated input signals. Variations occur due to the fact that transmission signals from stations operating at the same power level may not reach the receiver with the same power because of differences in transmission distances, carrier frequencies, atmospheric conditions, and obstructions between the transmitter and receiver.
Digital television receivers, which are susceptible to the same signal magnitude variations as the standard NTSC-type television receivers, need to maintain the received signal at a specific power level in order for proper processing to occur. Digital transmission signals, however, differ from standard analog transmission signals in that they appear to be a random sequence of pulses, resembling a random noise signal of varying amplitude. Some digital transmission signals, unlike analog transmission signals, lack a carrier signal that can be "locked" onto. Thus, it is difficult, if not impossible, to use standard AGC techniques effectively for a digital transmission signal. Moreover, the prior art automatic gain control circuits designed for digital transmission signals have been incapable of accurately demodulating the signal because their operation has been too varied.
Advanced digital television receivers are currently being designed to demodulate QAM, VSB, OFDM or QPSK encoded signals. In order to properly demodulate each of these encoding techniques, it is important to maintain the continuously varying signal levels at a particular power level. In addition, it is preferable that the AGC gain for modulated digital transmission signals be fast acting to increase accuracy and minimize the amount of time required to set the proper gain. Unfortunately, the current high gain AGC circuits tend to closely follow the pulsing data patterns contained in the received signal, causing the AGC circuit to fluctuate wildly.
Accordingly, what is needed is an AGC apparatus and method that can quickly adjust the gain while avoiding the problems caused by a fluctuating gain closely following the data patterns within the received signal.
It is, therefore, an object of the present invention to provide an AGC circuit having selectable slew rates for a quick signal lock-on and for accurate adjustment of the signal power without being effected by embedded data patterns in the received signal.
It is a further object of the present invention to provide a digital form variable slew rate AGC circuit having a first slew rate to provide a fast coarse adjustment of the signal power, and a second slew rate to provide a fine adjustment of the signal to the desired power.
It is still another object of the present invention to accomplish the above-stated objects by utilizing an apparatus which is simple in design and use, and economical to manufacture.
The foregoing objects and advantages of the invention are illustrative of those which can be achieved by the present invention and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the invention will be apparent from the description herein or can be learned from practicing the invention, both as embodied herein or as modified in view of any variations which may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel methods, arrangements, combinations and improvements herein shown and described.